The present invention relates to a guard ring structure for a reach-through type high voltage semiconductor device.
There are two conventional guard ring structures.
A first conventional guard ring structure has equally spaced four annular guard ring regions surrounding a main region located at the center. Spacings among the main region and the guard ring regions are all equal to a predetermined value L. Four equal inter-region spacings are; a first spacing between the main region and the first nearest guard ring region, a second spacing between the first nearest guard ring region and the second nearest guard ring region, a third spacing between the second guard ring region and the third guard ring region, and a fourth spacing between the third guard ring region and the fourth guar ring region. The main and guard ring regions are p type regions formed in an n type drift layer formed on an n.sup.+ type substrate layer. The main region forms a main pn junction with the drift layer. An anode electrode is connected with the main region, and a cathode electrode is connected with the substrate layer. The guard ring regions are connected with no electrodes.
The main and guard ring regions are formed by thermal diffusion from the semiconductor surface through diffusion windows. Hence, each diffused region has a cross sectional shape which is round at each corner. In the case of a reach-through type, a depletion region produced from the main junction by application of a reverse bias extends fully across the drift layer and reaches the substrate layer at a voltage level lower than a reverse bias voltage at which an avalanche breakdown occurs at the main junction. For example, the drift layer is formed to have a thickness of about 50 .mu.m and an impurity concentration of about 1.times.10.sup.14 /cm.sup.3 to obtain a breakdown voltage of about 600 V.
The guard ring structure functions as follows:
The anode electrode 20 is grounded, and a positive potential is applied to the cathode electrode 21. Under these conditions, the junction between the p type main region and the n type drift region is reverse-biased, and a depletion layer extends into the drift region 5 whose impurity concentration is low to obtain a high breakdown voltage. The junction between the main region and the drift layer has a curved portion as well as a planar portion. The curved junction portion has a higher field intensity than the planer portion. Therefore, without the guard ring regions, an avalanche breakdown would occur at the curved junction portion at a reverse bias voltage lower than a breakdown voltage expected from the planar junction.
The first guard ring region near the curved main junction portion mitigates an increase of the electric field in the lateral direction when the cathode potential increases and the depletion region from the main region reaches the first guard ring region. With a further increase in the cathode potential, the depletion region starts extending from the first guard ring region. In this way, the guard ring structure acts to reduce electric field crowding, and to improve the withstand strength.
The equally spaced multi guard ring structure is easy in design, and capable of making a maximum field intensity of a surrounding guard ring region lower than a maximum field intensity of an inner region. The maximum field intensity of the first guard ring region is lower than that of the main region, and the maximum field intensity of the outer guard ring region is lower than that of the inner guar ring region. The outermost guard ring region is an exception. However, the guard ring structure is arranged to have such properties by increasing the number of guard rings or other measures.
When the spacing between the main region and the first guard ring region is reduced, the effect of mitigating the field strength at the curved main junction portion is increased, and the region at which the electric field is maximum in the main region is shifted in the curved portion toward the planar junction portion. Therefore, a reduction of the spacing between the main region and the first guard ring region improve the breakdown voltage of the main junction. However, too narrow a spacing causes a disadvantageous effect as explained later.
A second conventional multi guard ring structure is shown in B. J Baliga "MODERN POWER DEVICES", John Wiley & Sons. Inc., page 99, FIG. 3.26. In a p type drift layer, there are formed an n.sup.+ type main junction region, an n.sup.+ type first guard (or field) ring region surrounding the main junction region with a first spacing from the main junction region, an n.sup.+ type second guard ring region surrounding the first guard ring region with a second spacing from the first guard ring region, and an n.sup.+ type third guard ring region surrounding the second guard ring region with a third spacing from the second guard ring region. In this structure, the guard ring spacing together with the guard ring width is decreased with an increase in the distance from the main junction region. This structure causes a depletion region to extend gradually from an end portion of the main junction region in the lateral direction. The guard ring width is decreased in the lateral direction away from the main junction region so that an outer guard ring is narrower than an inner guard ring. Therefore, the depletion layer depth is gradually decreased along the lateral direction away from the main junction region, underneath the gradually narrowed guard ring regions. This design can save space at the device periphery. As explained in this document, the guard ring regions share the applied voltage equally in the ideal case, and avalanche breakdown is produced at the outer edges of all the guard ring regions simultaneously. However, this structure is a non-reach-through structure in which a depletion layer from the main junction region does not reach the bottom of the p type drift layer.